Why can't I UV unwrap a rectangle in cycles?
This isn't an error so much as it is a warning.  The unwrap should have still completed (open the UV/Image Editor to check).  This message just means that you have scaled the object more in one direction than others.

When you scale an object in object mode, it is not really "applied" to the object's geometry. I like to think of scaling as a modifier that is always last in the stack, whatever you do in edit mode (or most other places) is as if it was done before the object was scaled.  So if you unwrap a non uniformly-scaled object it act like it was unwrapped before scaling, meaning the textures may be stretched or distorted.

To fix this just go into object mode and hit Ctrl+A > Apply Scale.  This will reset all the scale values to 1 and apply the scaling to the mesh itself.